test: follow Groonga's change

Because Groonga 4.0.6 has been fixed a bug that similar search
doesn't use IDF (Inverse Document Frequency) for choosing
target tokens. In the previous versions, it used token ID
instead of IDF. [groonga-dev,02731] [groonga-dev,02754]

NOTE:
  * Should we fix data instead of assertions?
  * Should we support the previous versions for the tests?